Audio signals, like speech or music, are encoded for example to enable efficient transmission or storage of the audio signals.
Audio encoders and decoders (also known as codecs) are used to represent audio based signals, such as music and ambient sounds (which in speech coding terms can be called background noise).
An audio codec can also be configured to operate with varying bit rates. At lower bit rates, such an audio codec may be optimized to work with speech signals at a coding rate equivalent to a pure speech codec. At higher bit rates, the audio codec may code any signal including music, background noise and speech, with higher quality and performance. A variable-rate audio codec can also implement an embedded scalable coding structure and bitstream, where additional bits (a specific amount of bits is often referred to as a layer) improve the coding upon lower rates, and where the bitstream of a higher rate may be truncated to obtain the bitstream of a lower rate coding. Such an audio codec may utilize a codec designed purely for speech signals as the core layer or lowest bit rate coding.
An audio codec can also adopt a multimode approach for encoding the input audio signal, in which a particular mode of coding is selected according to the channel configuration of the input audio signal. Switching between the various modes of operation requires the provision of some sort of in-band signalling in order to inform the decoder of the particular mode of coding. Typically, this in-band signalling may take the form of mode bits which require a proportion of the audio payload format which therefore consumes transmission bandwidth.
Additionally, the audio payload format may need to have the provision for supporting future changes to the multimode audio signal format whilst still maintaining the ability to cope with legacy modes of coding.